Separator membranes are required to insulate the anode and cathode electrodes in storage batteries during the full range of operating conditions—from low temperature to high temperature, and across a wide range of charging and discharging rates. The membranes are necessarily thin and microporous to maximize the flow of ions during charging and discharging of the batteries.
Lithium ion batteries are noted for their superior performance, except for rare occurrences of shorts or other conditions that can cause overheating, overpressure and fires. Shorts may be caused by dendrites on electrodes growing through separators, or by presence of conducting particulates from cutting of foil electrode collectors.
Conventional separators for lithium ion batteries are constructed from non-crosslinked polyolefins or from fluoroplastics. Polyolefin separators have been found to respond adversely in the presence of electrolyte solvents at temperatures above 60° C. For example, when investigated for restrained shrinkage characteristics by placement within an embroidery hoop and exposed for 1 h to propylene carbonate, polyolefins have been observed to split or to develop pinholes.
A claimed safety feature for certain commercial polyolefin separators for lithium ion batteries is the use of a thermoplastic additive that is expected to melt and form an electrically insulating film at a melting temperature below that of the separator. This so-called “shut-down separator” has failed many tests in batteries where overheating continued to where fires ensued. An inherent difficulty with this process is the low surface tension forces of the thermoplastic additive, which have little thermodynamic motive for forming a film to displace the electrolyte that has previously been in contact with the principal polymer of the separator.
Conventional fluoroplastic separators (polyvinylidene fluoride (PVDF) and/or copolymers of polyvinylidene fluoride and hexafluoropropylene) have been observed to gel and dissolve completely in bulk propylene carbonate at temperatures around 60° C. When confined between the electrodes of a lithium ion battery where there is a limited amount of solvent, these polymers may become gels, but may continue to perform at somewhat higher temperatures. However, they are structurally weak gels having little resistance to growth of penetrating conducting dendrites that sometimes occur in batteries.
Microporous membranes made of alternative polymers such as polysulfones, and acrylonitrile-butadiene-styrene are also known to be soluble in hot electrolyte solvents such as propylene carbonate.
Effective electrode separators for batteries are further thin, porous, non-conducting to electricity, and non-degrading in electrical fields up to 4.5 volts; in addition, they must be chemically durable and physically durable in electrolyte solvents at temperatures greater than 90° C. To date, no separator has met all these criteria at a marketable cost. Polyolefin separators dominate the market, but they lack the porosity for high current-density and, as stated hereinabove, have limited durability in the presence of electrolyte solvents at temperatures above 60° C. Separators made from polyvinylidene fluoride and co-polymers are being developed with some, but not all, of these attributes, but high cost and high polymer densities remain major limitations.